This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. ECD studies of two modified Amyloid beta peptides (20-29 and 25-35) were performed to investigate the role of H+ radicals in the ECD of peptide ions and the free radical cascade (FRC) mechanism. 2,4,6-trimethylpyridinium (TMP) was used as the fixed charge tag, which has the potential of simultaneously trapping the originally formed radical upon electron capture and inhibiting the H+ generation. Unlabeled, singly-labeled, and doubly-labeled peptides were each analyzed by ECD. It was found that both the number and locations of the fixed charge groups influenced the backbone and side-chain cleavages of these peptides in ECD. The frequency and extent of backbone cleavages decreased and those of side-chain cleavages increased with the addition of fixed charge tags. A doubly-labeled peptide with tags spaced far apart produced fewer multiple side-chain cleavages, but slightly more backbone cleavages than the one with neighboring tags. Despite the non-protonated nature of all charged sites in doubly-labeled peptide ions, several low abundance c and z+ ions were still observed in their ECD spectra. Thus, while H+ transfer may be important for the N-C(alpha) bond cleavage, there also exist other pathways. Finally, it appeared that the presence of this particular radical trap merely inhibited but did not eliminate the FRC pathway which most likely proceeded via H+ abstraction through space and produced numerous sidechain and backbone cleavages. This study has been recently published (Li et al., 2008) in the Journal of the American Society for Mass Spectrometry, 2008, 19, 1514-1526.